Many applications require laser sources that offer a narrow linewidth, a high degree of coherence and low optical phase noise.
Long baseline interferometry, for instance, rely on highly coherent lasers to detect extremely small length changes over tens and hundreds of kilometers by observing the change of phase of an optical beam after propagating through that distance. Gravitational wave detectors, formation-flying satellites and length-stabilized fiber distribution systems also uses such techniques. A laser source possessing a coherence length much longer than the path length is required in order to produce interference fringes to be detected with a good signal to noise ratio.
Microwave and millimeter wave generation is another application where two low phase noise optical beams are combined within a high speed photodetector or a photomixer to produce an RF signal at the frequency difference between the two beams. Since it is relatively easy to frequency tune lasers over hundreds of GHz, widely tunable RF generators can therefore be achieved. Since the phase noise of the RF carrier is the sum of the uncorrelated phase noise of both lasers, it is important to have lasers with maximum coherence and minimum phase noise.
Some types of LIDARs such as DIAL (Differential absorption LIDAR) and Doppler LIDAR also rely on the heterodyne detection of the reflected laser signal (delayed pulses or phase shifted cw) with the local laser oscillator in order to detect the frequency shifts induced by moving or vibrating remote objects. Here again, the signal to noise ratio of the received signal will depend on the degree of coherence of the laser pulse after its round trip.
Some types of fiber sensors also require narrow linewidth light source to perform sensitive measurements with high signal to noise ratio. Applications such as hydrophonic or seismic detector arrays, distributed temperature and pressure sensors, or structural monitoring for buildings, bridges, roads, ships and aircraft can all benefit from a narrow-linewidth laser source.
Narrow-linewidth lasers are often also used as seed lasers to synchronize more powerful lasers or amplifiers which inherit the linewidth and coherence of the seeder laser.
For those applications and many more, the use of high spectral purity (extremely narrow linewidth) lasers is mandatory. Linewidth lower than a few kHz is often required in many applications. In some cases, the lasers must be installed on moving platforms, aircrafts and satellites where size, weight, power consumption, robustness and reliability constraints are of prime importance. For applications where a large number of narrow linewidth lasers are required, cost is also a major issue. Some applications may also require the laser output to be provided with specific optical polarizations in polarization-maintaining fibers.
Different types of narrow linewidth lasers are currently available to the market. These are described below.
YAG lasers are bulk crystal, optically or electrically pumped, lasers. In some of their realizations, they offer relatively high power and narrow linewidth (5 kHz) but are bulky and relatively power inefficient. Single mode operation is also difficult to sustain without requiring the addition of special external controls like magnetic field, strain, temperature and so on.
Fiber lasers offer a very interesting alternative to these solutions. The most attractive designs are built with Bragg gratings written in or around an Erbium doped amplifying fiber pumped by one or more semiconductor lasers. They offer natural linewidths below the kHz level and are built entirely out of fiber. They are lightweight and compact, and can provide large amounts of output optical power. One drawback is that they require specialty fibers with gratings, which can be expansive to produce and may not be well suited for some space application due to radiation sensitivity. Another drawback comes from their sensitivity to environmental perturbations like vibrations, acoustic noise, strain or temperature. Their linewidth, even if it is small, is not sufficient for applications where coherence length grater than 50 km is required. Other types of fiber laser use optical resonators, like ring cavity or Fabry-Perot, with optical filters to select one emission wavelength. These lasers are even more difficult to put under stable and reliable operation mode. Since fiber lasers rely on a high power semiconductor laser to pump the active Erbium-doped fiber and do not use the output of the laser directly, these laser sources tend to be more complex, more costly, less reliable and less efficient than the simple semiconductor lasers they are made of.
It would be extremely advantageous to use telecommunication-grade semiconductor lasers as laser sources for the above-mentioned applications. Indeed, such lasers benefit from many years of intensive developments efforts that have been invested over the last decades to produce low cost, reliable optical sources for the telecommunication industry. Mass production of such lasers allow them to be available at a very low cost. Mature laser architecture and packaging techniques make these lasers extremely reliable. Many lasers comply with the Telcordia standards which ensure the performance of the laser for decades in harsh environmental conditions. A number of space-qualified semiconductors are also already available. Modern semiconductor lasers can provide output power levels exceeding 50 mW, and new generation of lasers can provide many hundreds of milliwatts, which is sufficient for many applications.
Unfortunately, although semiconductor lasers have a number of highly desirable characteristics for the targeted applications, their linewidth is often too large to be useful. Indeed, the linewidth of semiconductor lasers is typically greater than a few hundreds of kilohertz, which makes their coherence length too short and their phase noise too high for applications requiring high spectral purity sources.
Various techniques have been developed to enhance the spectral quality of semiconductor lasers. External cavity lasers (ECL) rely on a mechanical cavity to feedback light in the active region of a semiconductor laser in order to reduce the linewidth of that laser. The mode-hop free operation of laser with such an optical feedback depends on the mechanical stability of the optical set-up, which may be difficult to maintain. The typical linewidth of such lasers is typically 100 kHz or less when measured over short periods of time, and increase over longer measurement periods due to the laser's inherent frequency sensitivity to acoustic and mechanical noise. This linewidth is not sufficiently narrow for high precision applications. Laser based on micromechanically-tuned cavity offer better robustness and immunity to vibrations, but their linewidth is still generally beyond the kHz level.
Other techniques which do not rely on alignment-sensitive optical have been demonstrated to reduce the natural linewidth of semiconductor lasers. One of these consists in using a highly sensitive optical filter used as a frequency discriminator to measure the frequency fluctuations of the laser, and to apply an electrical signal back to the laser to tune its frequency so as to compensate these optical frequency fluctuations. This frequency locking system essentially transfers the short-term frequency stability of the frequency discriminator to the laser. If the optical frequency discriminator has a low frequency noise in the locking bandwidth, the laser will inherit this low noise and will display a narrower linewidth.
Such a linewidth reduction system is demonstrated by Ohtsu, M. and N. Tabuchi in “Electrical Feedback and Its Network Analysis for Linewidth Reduction of a Semiconductor-Laser.”, Journal of Lightwave Technology Vol. 6, No 3, pp-357-369, 1988. In these experiments, the light from a semiconductor laser is sent through a free-space Fabry-Perot resonator used as frequency discriminator that converts the frequency fluctuations in intensity variations which are measured by a photodetector. A specially-designed feedback circuitry, which takes into account the frequency tuning response of the laser, imposes frequency corrections to the laser by changing its injection current. The drawback of this approach is that the Fabry-Perot resonators are bulky devices that are susceptible to acoustic and mechanical noise and require optical alignments. Such a frequency discriminator is therefore not appropriate for use in mobile platforms or noisy environments.
Solid-state, all-fibers Mach-Zehnder interferometers are common devices that offer sharp frequency response and that can be used as more convenient frequency discriminators than free-space Fabry-Perot resonators. Such interferometers can be made small, lightweight and low-cost. U.S. Pat. No. 6,891,149 granted to Lewis et al. describes a frequency locking system using such Mach-Zehnder interferometer to generate the electrical feedback signal to reduce the frequency noise of a laser source. Such a frequency locking system can advantageously be used with semiconductor lasers to reduce their linewidth. A similar system was shown by Cranch, G. A. in “Frequency noise reduction in erbium-doped fiber distributed-feedback lasers by electronic feedback.”, Optics Letters Vol. 27, No 13, pp-1114-1116, 2002, to reduce the linewidth of a DFB fiber laser. Both solutions rely on the use of a dual output Mach-Zehnder interferometer with two photodetectors connected so as to measure the differential power in the two output branches to generate a frequency error signal. These dual photodetector systems cannot make use of the monitoring photodetector that is often already included in telecommunication semiconductor lasers, therefore uselessly increasing the component count. The proposed dual-photodetector systems are not designed to use other types of other frequency discriminators such as all-fiber Michelson interferometers, which may be more advantageous to reduce the fiber length and reduce mechanical sensitivity.
Therefore, there is a need for a low-cost semiconductor laser source that can offer narrow linewidths while being power efficient, compact, lightweight, and highly reliable. Moreover, to overcome the disadvantages of the prior art devices, it would be desirable that they use the minimum of components, the smallest frequency discriminators, and be as immune to external perturbation as possible. It would also be desirable that they provide the maximum performance that can be achieved by using low noise current source, and take into account the frequency noise spectrum of the laser and frequency discriminator, and the tuning response of the laser. It would also be desirable that they operate automatically without user intervention.
It would also be desirable to provide a narrow linewidth laser that can be continuously frequency-tuned over a large frequency range.
It would also be desirable to provide a high coherence light source whose frequency is extremely stable or is known with high accuracy.